In a CDMA mobile communication system, the system performs a location registration process to recognize where a mobile station is located before the mobile station enters the standby status. Even in the standby status, this process is performed, for the center to control the movement of the mobile station, periodically or every time the registered area for a location is changed.
It happens in some areas where the mobile station is located, depending on the environments for radio wave propagation, that forward signals from a base station reach the mobile station easily whereas reverse signals from the mobile station do not reach the base station well. When the mobile station is located in such a area, it is difficult for the location registration request signal from the mobile station to reach the base station, and the base station can not return the signal to acknowledge the reception of the location registration request signal, causing the mobile station to repeat the process of the location registration. In the CDMA mobile communication system, therefore, the power is continuously consumed while waiting for the acknowledge signal because the receiving status is maintained after the location registration request signal is transmitted.
FIG. 6 specifically illustrates the flow of this location registration process. First, the mobile station acquires a base station (step S180). This is a process in which a base station that transmits a signal having a receiving electric field of a level higher than a predetermined level is sought for. When a base station that fits the condition is not acquired, OUT OF AREA is displayed (steps S182 and S184). After OUT OF AREA is displayed, the timer is set (step S188), and another attempt to acquire a base station is made (step S180) after the timer has counted a predetermined period of time (step S190).
When signals that have a receiving electric field of a level higher than a predetermined level are received, a base station providing high receiving quality is acquired, and the signals from the acquired base station are received (step S186). Then predetermined information is extracted from the received signals, and it is determined whether the location registration process is necessary (step S192). If it is determined that the location registration process is not necessary, the standby status is set (step S194). If it is determined that the location registration process is necessary, the location registration request signal is transmitted to the base station (step 196). When the location registration request signal is received by the base station, the acknowledge signal is usually transmitted from the corresponding base station, and the mobile station waits for the acknowledge signal for a predetermined period of time (step S198).
When the acknowledge signal is returned within the predetermined period of time, the location registration is completed and the standby status is entered (step S194). When the acknowledge signal is not received within the predetermined period of time, the location registration request signal is transmitted again after acquiring the base station, and the mobile station waits for the acknowledge signal from the base station. If the mobile station moves only a little under such a situation, the location registration request signal does not reach the base station making it impossible for the base station to transmit the acknowledge signal. Consequently, the mobile station has to continue the location registration process solely waiting for the acknowledge signal.
Thus, in those areas where the reverse signal from the mobile station does not reach the acquired base station, the mobile station has to wait for the acknowledge signal from the base station for a long period of time significantly consuming the batteries.
When a mobile station of the prior art is located in an area where the forward signal reaches the mobile station but the reverse signal does not reach the base station as mentioned above, the base station can not transmit the acknowledge signal to the mobile station because the location acknowledge signal does not reach the base station. If the acknowledge signal is not received after waiting a predetermined period of time, the mobile station acquires the base station again and requests the location registration, keeping waiting for the acknowledge signal and consuming the power.